Induction, here, connotes the manifestation, by pre-committed cells, of immunogenetically-defined gene products composing a surface phenotype. Data from a battery of T and B lymphocyte induction assays in vitro imply the following: (1) Thymopoietin (Tp) is a natural inducer of prothymocytes. (2) Tp inhibits induction of certain B cell phenotypes. (3) Tp receptors on committed T and B inducible cells are geared to different internal mediators, cAMP vs cGMP, hence reciprocal regulation. (4) Tentative evidence of selective induction, so far known only for Tp, by bursapoietin (Bp) suggests the validity of Bp as a counterpart reciprocal regulator. (5) Ubiquitin, a fully-sequenced NON-selective inducer that is present but confined in all nucleated cells, uses different receptors, and is inductive or inhibitory according to its concentration. (6) Immunotherapeutic indications from work with ill-defined or untitrated biomaterials are dangerous, see points (1-5); the assays described set new standards. (7) Events and sequence of TL synthesis by thymocytes and TL plus leukemia cells, deduced from 35S-Met-labeling, set the stage for investigation of TL induction in prothymocytes. Proposed: (a) Further immunogenetic definition of prothymocyte surface phenotype(s). (b) Full-scale analysis of Bp as a counterpart of Tp; with emphasis on conditions to disclose reciprocal regulation (inhibition of T-induction). (c) Similar approaches to TCGF and LAF, which complete the panel of 5 agents for study. (d) To determine whether the 5 agents have additional actions that precede induction (recruitment from stem sets) or follow it (effector function). (e) From the base (7) above, to investigate, by internal labeling, the induction of TL in and on prothymocytes. (f) Supplementing (e), to probe induction by examining also its converse, inhibition; a possible model being inhibition, by TP5, of induced expression of cIg and sIg by Ig-inducible 70Z/3 cells.